Powder metallurgy



July 26, 1960 w. R. RAYMQNT POWDER METALLURGY Filed Aug. 10. 1955 Ala/ANA? RAY/V047 Figure 2 is an isometric view of a porous refractory moldassembly for the preform of Figure 1 showing infiltrant metal particlesfor the infiltration operation of this invention.

Figure 3 is a vertical cross-sectional view alongthe line III-Ill ofFigure 2 with the particles omitted from the mold cavity.

Figure 4 is a vertical cross-sectional somewhat diagrammatic view of themold of Figure 2 mounted in a vacuum furnace for the heat treatingoperations.

Figure 5 is a view similar to Figure 3 and illustrating the assemblyafter the heat treatment in the furnace of Figure 3.

Figure 6 is a perspective view of a finished turbine bucket produced bythe method of this invention.

As shown on the drawings:

In Figure 1 the die assembly -10 includes a pair of opposed punches ordies 11 slidably guided in a fixture 12 and having active facescooperating to produce a preform 13 of the desired airfoil contours offluid directing members.

The material used to form the preform 13 may be a powder of any of avariety of ceramic, intermetallic or other refractory compositions suchas for example, alumina, titanium carbide, zirconium boride, tungstencarbide and the like. It is preferred that particle size of thesepowders be relatively small and distributed more or less uniformly overa desired particle size range. A particularly effective particle sizedistribution pattern for titanium carbide includes about 35 parts byweight having a maximum dimension of 3 microns, about 32 parts by weighthaving a dimension in the range of from 3 to 6 microns, about 30 partsby weight having a dimension in the range of from 6 to 12 microns, and amaximum of about 3 parts by weight having a particle size in excess of12 microns.

While it is not absolutely essential that the refractory particles bepure, better results will be obtained if certain contaminants are heldwithin reasonable limits. For example, the specifications for thetitanium carbide which are employed this process are substantially asfollows:

Table l Ingredient: Percent by weight Combined carbon minimum Freecarbon ..maximum.. Iron do Oxygen d 1.2. Nitrogen do 0.25 Hydrogen do0.03 Other impurities ..-do 0.75

The procedure for producing the preform may vary. One preferredprocedure consists in mixing the refractory particles with a thermallydepolymerizable binder such as polybutene, the binder constituting fromabout 5 to 35% by volume of the compact. Normally, the binder is addedin solution in a suitable solvent such as xylene. The preform is thenshaped cold in the dies 11 at pressures of about 0.5 to 25 tons persquare inch and heated to a temperature sufficient to depolymerize thebinder, and drive off the solvent. Processes of this type are fullydescribed in US. Patent No. 2,593,507 to Eugene Wainer.

Another technique includes formation of a press block of the powder inthe dies 11 at die pressures in the range-of 0.5 to 50 tons per squareinch followed by presintering of the block in a vacuum furnace having apressure of from 0.1 to 500 microns of mercury. The pre-sintering isconducted at temperatures of from 2000 to 2650 F. for a period varyingfrom 5 minutes to 5 hours. The die block is then machined to the desiredcontour or alternately, of course, could be die shaped as accurately aspossible.

According to this invention the preform 13 is to be infiltrated and heattreated in an inert porous mold which will impart finished surfacecharacteristics to the blade. While a number of mold materials areuseful, zirconium oxide, stabilized against crystallographic changes, ispreferred. A heat stabilizer such as calcium oxide which reacts withzirconium oxide to form a stabilized crystallographic material, is used.Normally, about 1% or less of calcium oxide will be sufiicient tostabilize the zirconium oxide at any temperatures reached during theheat treatment and infiltration of the preform.

It is preferred to have the zirconium oxide particles more or lessuniformly distributed in the range of from between 5 and 44 microns. Toaid in shaping the mold, a lubricant such as calcium stearate or leadstearate in an amount of 1 to 5% of the total composition can be usedtogether with a binder such as methyl cellulose. About 1 to 2% by weightof a methyl cellulose solution having a 5% concentration in water willnormally be suflicient.

Pressures employed in shaping the mold in metal dies (not shown) mayvary widely but usually pressures of about '1 to 5 tons per square inchwill be satisfactory.

The green mold is fired at temperatures of from 2000 to 3000 F. for aperiod of from 30 minutes to 5 hours. Usually a 2-hour firing treatmentat 2500 F. is preferred. As explained above, the firing can occursimlultaneously' with the sintering of the preform in the mo d.

As shown in Figures 2 and 3, the refractory mold is illustrated as avertically split mold 14 composed of halves or sections 15 and 16together cooperating to define a mold cavity 17 which snugly receivesthe vane portion of the preform 13 while a rounded head 13a of thepreform projects into an enlarged cavity portion 17a. This cavityportion f17a communicates with a gate passage 18 projecting laterallyfrom an end of the cavity 17a to a sprue cup 18a alongside the mold andhaving The mold sections 15 a cavity feeding the gatepassage. and 16 areheld together in any suitable manner as by means of clamps, insertion ina sheath, or the like.

If desired, the preform 13 can be made directly in the mold. Thus thepowder can be incorporated in a suitable slurry which is then slip castinto the porous mold which will drain off the liquid components of theslurry and confine the solids in the shape of the mold cavity.Alternately, the powder can be centrifugedin the mold to form thepreform.

7 As shown in Figures 2 and 4, the mold 14 with the preform 13 thereinhas infiltrant metal particles 19 de-. posited in the sprue cup 18a andsurrounding the preform end 13a. The mold assembly is now ready for theinfiltration step and is placed in a sealed furnace 20 which can beevacuated or flooded with an inert gas such as argon or helium tomaintain an inert atmosphere around the mold. The furnace is heated asby means of electrical heating elements 21 which surround the mold 14.

The infiltration is carried out at temperatures ranging from aboutmelting point of the infiltrant metal to about 200 above that meltingpoint. The infiltration step will be completed in a time period. from aslittle as 5 minutes to as much as 2 hours or more.

In addition to providing a sufficient amount of the corrosion resistantmetal to impregnate completely the pores of the porous preform. 13,additional amounts of the infiltrant metal are provided to form a castroot 22 (Fig. 5) for the turbine blade, the root completely envelopingand bonded to the anchoring end 13a formed on the preform 13. The metalof the root 22 and the infiltrant of the pores of the preform 13 therebyprovide a continuous, mono-metallic single phase system which not onlyprovides the strength and corrosion resistance required in turbineblading or the like but also provides for a permanent bond between thevane portionand the root portion of the turbine blading.

Numerous difierent infiltration metals can be employed 5; to advantagein the present invention. resistant nickel-chromium.alloys-and thecobaltbase alloys are particularly valuable for this purpose. The-commercialheat resistant alloys such as thoseof the In! cone and heat resistantnitridedstels (Nitralloy) may also be employed. A typical lnconel alloy:(fInconel X) has the following composition; l.

Table 11 Element: Percent by weight C 508 maximum Mn .05 to 1 Si .06maximum C1 1 4-1 6 Al 0.5-1.0 Ti 2.0-2.6 C DEB-1L2 Fe 6-8' Ni BalanceAfter the infiltration has been completed, the infiltrated comp-act canbe further heat treated in the mold. For example, this can beaccomplished byzmerelyftlroppjng the temperature of the assemblyfrornthe-infiltration temperature to a temperature which will normally he on:the order of 200 F. or so below the melting point of the infiltrant.Again, the heat treatment time will vary considerably depending upon thematerials employed, the strength desired, the porosity, and similarfactors, but ordinarily periods ranging from minutes to 2 hours will beemployed. The heat treatment, like the infiltration, is carried outunder non-oxidizing conditions, and preferably under vacuum conditionsin which the absolute pressure is in the range from about 0.5 to 500microns 'of mercury.

As shown in Figure 6, a turbine bucket 23 formed according to thisinvention has a vane portion 23a of airfoil shape and an enlargedmassive anchoring root end portion 23b. The vane portion 23a is composedof a skeleton network or matrix of the refractory compounds with thepores of the network or spaces between the particles filled with theinfiltrant metal in firmly bonded relation thereto. The root end 23b iscomposed of the infiltrant metal although it also has a core of therefractory compound surrounded by the infiltrant metal. All surfaces ofthe bucket 2-3 are smooth and have imparted thereto a finish of thewalls defining the mold cavity. Since the mold material is not wet bythe infiltrant metal and since the mo-ld is quite porous, the metal canfreely flow to all surfaces of the bucket without causing the bucket tostick to the mold.

According to this invention it is also practical to form the mold in onepiece around the preform and to simulaneously pre-sinter the preform andfire the mold. This will prevent relative shrinkage between the preformand h mold so that stresses are minimized. In this modification thepreform can be completely enveloped by the mold. This modification hasthe advantage of eliminating separate firing and sintering steps andalso eliminating the necessity for machining the end of the root whichwould otherwise be exposed in the sprue.

In order to further control the shrinkage, the particle sizes andrelative densities of the preform and mold canbe regulated. For example,suitable shrinkage conditions are obtained by making a preform oftitanium carbide powder of less than 5 micron particle size under apressure of 1 ton per square inch at room temperatures. The powder cancontain about 1% lubricant or plasticizer such as Sterotex or paraffinwax. The preform thus formed has a density of about 50%. i

The mold is formed from zirconium oxide powder of about 20 to 40 micronsize, is pressed at room temperature at pressures of about 1 ton persquare inch and may have a lubricant and a binder added. A suitablelubricant is about 3% by Weight Sterotex. A suitable binder is about 2%by weight of a 5% solution of methyl The rcorrosion causes asimultaneousshrinkage of. thenrold'randfpreform in amounts of 7 to 8% byvolume and :theresulting parts will have .ailensity of about623to=64-%-. .The-sint'ering and firing can occur simultaneously withthe preform the mold or'separated from the mold.

.If the mold is formed around the preform, ,provision must be made tocontact the preform with the infiltrant metal. The infiltration can thenoccur at-temperatures of about 2600 F. V

If shrinkage of the mold and preform are notcorrelated, the mold cavityshould be shaped so that-itwill permit relative shrinkagewithout-stressing thepreform to a rupture point. The pre-sintering andpre firing of the mold will bring the parts down to a common remaining,shrinkage factor where they will shrink at substantially equal rateswhen heated-during the subsequent infiltrating and sinteringoperations.

Carrying out theinfiltration under vacuum conditions takes advantageofthe improved properties of alloys melt: ed under vacuum conditions ascomparedSWith air melted alloys. For :example, a typical corrosionresistant, alloy has a ductilityabont four times as great whemmeltedj'invacuum as compared to its ductility whenmelted 'in, air.

The following physical properties were obtained by the process of thepresent invention employing titanium carbide particles as the matrix andInconel X as the infiltrant metal:

Table III Density 6.2 gms./cc. Tensile strength, room temperature 50,000p.s.i. Modulus of elasticity at 1800 F... 40,000 p.s.i.

Stress rupture strength:

hr. life at 1600 F 40,000 p.s.i. 100 hr. life at 1800 F 15,000 p.s.i.Thermal expansion, per F.:

From 70' to 1200 F 5.5 10- in./in. From 70 to 1800" F 6.0 10- in./in.Thermal conductivity 0.063 .cal./sec./cm.

C./cm. Electrical resistivity 1.37-1.43 10 ohm/ cm. Impact strength,unnotched Izod--. 8-10 ft. lbs. Hardness 55-58 Rc. Weight gain, after100 hrs. in still air at 1800 F 20-30 mg./cm. Transverse strength:

Room temperature- 190,000-250,000 p.s.i. 1600 F 150,000-190,000 p.s.i.1800" F 115,000-150,000 p.s.i.

It will be'evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

I claim as my invention:

1. The method of making corrosion resistant shapes from refractorycompositions which comprises forming a refractory powder into aself-sustaining shaped preform, supporting said preform in aporousceramic mold Which has a thermal shrinkage rate substantially the sameas that of said preform, infiltrating said preform while in said moldwith a molten corrosion resistant composition under non-oxidizingconditions, and heat treating the infiltrated preform while the same isstill confined in said mold.

2. A method of making corrosion resistant shapes from refractorycompositions which comprises forming refractory powder into aself-sustaining shaped preform, supporting said preform in a tightfitting complementary ceramic mold having a thermal shrinkage ratesubstantially the same as that of said preform, infiltrating saidpreform while in said mold with a molten corrosion resistant compositionunder vacuum conditions, and heat treating the infiltrated preform whilethe same is still confined in said mold.

3. The method of making improved corrosion resistant shapes fromrefractory compositions which comprises forming a refractory powder intoa self-sustaining shaped preform, confining said preform in a tightlyfitting ceramic mold having a thermal shrinkage rate substantially thesame as that of said preform, infiltrating said preform in said moldwith a molten corrosion'resistant infiltrant composition undernon-oxidizing conditions, reducing the temperature after completion ofinfiltration to a temperature below the melting point of the infiltrantcomposition but high enough to heat treat the infiltrated mass, and heattreating said infiltrated mass at said temperature. 4. The method ofmaking an infiltrated article having controlled surface characteristicswhich comprises compacting a.powder to provide a self-sustaining preformof desired shape, forming a complementary preform mold having ashrinkage rate substantially the same as the shrinkage rate of thepreform at the infiltration temperature, assembling the preform in themold, and infiltrating the preform at an elevated infiltrationtemperature with a material that will not wet the moldbut which iscompatible with the preform, allowing the preform and mold to shrink atsubstantially the same rate, and continually ca supporting the preformin the mold whereby the mold finish will be imparted to the preform andthe surface characteristics of the infiltrant material will becontrolled by the mold finish. r

5. The method of making an infiltrated powdered metal article whichcomprises compacting a powdered metal to form a self-sustaining preformof desired shape, forming a preform mold for said preform, regulatingthe particle sizes and relative densities of the preform and mold toprovide substantially the same shrinkage rate for the preform and mold,assembling the preform in the mold, contacting the preform with aninfiltrant metal which is compatible with the preform but which will notwet the mold, heating the assembly above the melting point of theinfiltrant metal to infiltrate the preform with the metal, allowing thepreform and mold to shrink at substantially the same rate, andcontinually supporting the preform in the mold to thereby control thesurface characteristics of the resulting article. V

References Cited in the file of this paten UNITED STATES PATENTS rea g-

1. THE METHOD OF MAKING CORROSION RESISTANT SHAPES FROM REFRACTORYCOMPOSITIONS WHICH COMPRISES FORMING A REFRACTORY POWDER INTO ASELF-SUSTAINING SHAPED PREFORM, SUPPORTING SAID PREFORM IN A POROUSCERAMIC MOLD WHICH HAS A THERMAL SHRINKAGE RATE SUBSTANTIALLY THE SAMEAS THAT OF SAID PREFORM, INFILTRATING SAID PREFORM WHILE IN SAID MOLDWITH A MOLTEN CORROSION RESISTANT COMPOSITION UNDLER NON-OXIDIZINGCONDITIONS, AND HEAT TREATING THE INFILTRATED PREFORM WHILE THE SAME ISSTILL CONFINED IN SAID MOLD.